The present invention relates to a magneto-optical recording and reading-out method. More particularly, the present invention relates to a magneto-optical recording and reading-out method comprising irradiating beams to a tape-type re-writable and erasable magneto-optical recording medium having at least a magneto-optical recording layer on one side of a base film (substrate) having a thickness of not more than 50 .mu.m.
With increase of the amount of information in recent years, there is a strong request for an information recording and readout system which is capable of compact recording, storage and stable retention of massive amount of information from various sources such as documents, pictures and sounds.
Magneto-optical recording has a feature such as high storage capacity per unit area, i.e., high area storage density owing to use of beams closed down to the threshold of diffraction of approximately 1 .mu.m.
The conventional disc-type magneto-optical recording medium has an advantage that it is capable of random access to the recorded signal. Also, in this magneto-optical recording medium, since beams are irradiated to the magnetic layer via a transparent substrate having a thickness of approximately 1.2 mm, the dust and flaws on the irradiated surface of the substrate can hardly become a cause of error.
However, the disc-type magneto-optical recording medium is not high in storage capacity per unit volume, i.e., volume storage density, because of the relatively large thickness (1.2 mm) of the substrate. Even if the substrate is reduced in thickness for elevating the volume storage density, no drastic improvement of the volume storage density can be expected as far as the recording medium has a disc-like shape because the cartridge housing of the disc-type recording medium is incapable of extreme reduction of thickness.
In the case of magnetic tape, it is possible to significantly elevate the volume storage density owing to the structural features that the substrate can be reduced in thickness and the tapes can be housed in winding in a cartridge.
A tape-type magnetic recording medium is applied to digital video recording, etc., but its memory capacity is not remarkably large since the area storage density thereof is not so high as optical disc. Also, the tape-type magnetic recording medium is small in coercive force and high in saturation magnetization as compared with the disc-type magneto-optical recording medium, so that the tapetype recording medium has the problem that when it is rolled up and kept in storage for a long time, there may take place transfer of recording to cause deterioration of the recorded signal.
A write-once optical tape has been developed as a recording medium which has both the advantage of optical disc of being high in area storage density and the advantage of magnetic tape of being high in volume storage density. This medium such as write-once optical tape is remarkable for its high recording density and excellent storage stability, but it is merely capable of write-once and incapable of rewriting unnecessary information. Therefore, in case where an information becomes useless, the whole recording medium storing such information is treated as useless things. This becomes a problem not only from the aspect of economy but also from the viewpoint of waste disposal.
The magneto-optical recording system can be roughly divided into two types. One is a light intensity modulation in which magnetization of the medium is preset in one direction and recording is carried out by modulating the light irradiation intensity while applying a bias magnetic field in the opposite direction, and the other is a magnetic field modulation in which recording is conducted by modulating the direction of the magnetic field without modulating the light intensity.
In the magnetic field modulation, since the magnetization must be switched to high speed, it is necessary to miniaturize the magnetic head which generates the magnetic field and to position it closer to the recording medium.
However, in the recording of the magneto-optical tape, movement of the recording medium is relatively low in speed while the recording head is moved at high speed unlike disc recording, so that it is difficult to realize practical application of a recording method which requires high speed access of the magnetic head, such as the magnetic field modulation.
In the light intensity modulation, a process for arranging magnetization of the recording medium in one direction, that is, erasing or initialization is required before recording.
For example, in the case of magneto-optical disc, erasing is effected by continuous irradiation of beams while exerting a magnetic field in the direction opposite to the bias magnetic field for recording.
It is possible to conduct, also, the erasing treatment of magneto-optical tape, but the tape must be moved at high speed for effecting recording soon after erasing. This is actually impractical.
Thus, for recording and reading-out a rewritable tape-type magnetic recording medium by irradiating beams thereto, a recording and readout system of a magneto-optical tape capable of compact recording, storage and stable retention of massive information and also capable of as several times of rewriting has been required.
In view of the above, the present inventors pursued intensive studies for providing an information recording and readout system capable of realizing a very high volume storage density and re-writable on a tape-type magnet-optical recording medium, and as a result, the present inventors have found that by effecting initialization of a tape-type magneto-optical recording medium and erasing of information in the recording medium by a specified method, that is, making the recording medium run in front of at least the single pole of a permanent magnet or electromagnet without contacting the magnet, crossing at a substantial right angle to magnetic force lines, an excellent magneto-optical recording and readout system can be realized. The present invention has been attained on the basis of this finding.